A number of studies revealed that a point mutation at either position 12, 13, 59, or 61 of ras p21 proteins is associated with a fundamental change in their biochemical properties including their ability to transform cells. The main objective of this project is to study the conformational differences between non- transforming and transforming ras p21 proteins as well as their conformational changes upon binding to GTP. A few important observations concerning the conformational changes upon addition of GTP to synthetic N-terminal segments of ras p21 proteins appeared in the last report. Additional significant results are as follows: (1) Upon addition of either the glycine- containing (Gly-peptide) and valine-containing (Val-peptide) 34 amino acid residue peptides of the N-terminal segments of ras p21 proteins to the solution containing GTP or ATP, the line width of all three phosphorus-31 NMR resonance, alpha-, beta-, and gamma-phosphate, were broadened. Simultaneously, all three phosphate resonances shifted downfield upon binding with peptides. However, the degree of their shifts was somewhat different. Beta- and gamma-phosphate resonances shifted downfield noticeably, but the alpha-phosphate resonance was not shifted to any significant degree upon addition of either the Gly- peptide or Val-peptide. (2) It is known that magnesium ion plays an important role in binding guanine nucleotide to ras p21 proteins. Upon addition of magnesium ion to the mixture of the Gly-peptide with GTP, all of three phosphate resonances shifted further downfield without broadening their line widths. (3) The Gly-peptide, in contrast to the Val-peptide, catalyzes the hydrolysis of GTP.